Pulse width modulators convert an analog input signal into a sequence of pulses, the pulse length of which is proportional to the amplitude of the input signal. Due to the value discreteness of the output signal, pulse width modulation (PWM) provides for high efficiency of the power conversion and offers high linearity.
Conventional PWM modulators use a clock generator which predetermines the switching frequency of the PWM modulator. FIG. 1 illustrates a basic circuit for generating a PWM signal 1 from an analog input signal 2. To generate the PWM signal 1, a comparator 3 compares the input signal 2 with the triangular signal output by a triangular-signal generator 4. As long as the amplitude of the input signal 2 is greater than the amplitude of the triangular signal, the PWM signal has the signal value 1 and otherwise the signal value of the PWM signal is 0.
Furthermore, self-oscillating PWM modulators are already known. Self-oscillating PWM modulators are cost-effective since there is no clock generator. Self generation of the switching frequency can be achieved by feeding the PWM output signal back to the input of the PWM modulator.
One problem in self-oscillating PWM modulators consists in that variations of the switching frequency can occur. Variations of the switching frequency lead to distortions in the output signal of the PWM modulator. If a number of channels of a system are to be processed by means of a number of self-oscillating PWM modulators, additional problems arise due to the lack of synchronism of the self-oscillating PWM modulators.
It is already known, therefore, to synchronize a number of self-oscillating PWM modulators. Synchronization is effected by coupling the self-excited oscillations in the respective PWM modulators. The coupling can be carried out, for example, via the output signal (i.e., e.g., via the load) or via coupling to internal nodes of the self-oscillating PWM modulators. Synchronized, self-oscillating PWM modulators enable, e.g., a three-valued pulse-width modulated signal to be generated by combining the two two-valued PWM output signals or the amplification of multi-channel audio signals.